This invention relates to isolation bearings for man made structures and more particularly relates to such bearings used for isolation of earthquake shock and vibration to protect such structures.
Bridges and building structures, when subjected to earthquake movements, often incur significant structural damage. Much of this damage can be avoided if the structure is supported by bearings which allow the superstructure to move relative to the bearings. For instance, if a bridge superstructure rests upon sliding bearings, all ground motion will not be transferred to the superstructure. Decoupling of the structure from the ground is called base isolation.
Two key parts of an effective base isolation system is a damping force element and a restoring force element.
Bearings or supports for accommodating movements of superstructure due to thermal and other forces have been known for some time. Such supports in the form of sliding bearings are, for example, described in U.S. Pat. Nos. 3,806,975; 3,921,240. Additionally, lead-rubber bearings for earthquake isolation have been known, see e.g., U.S. Pat. No. 4,117,637. Sliding bearings, usually constitute support pads which under stress are free to move translationally or rotationally between a base support and the superstructure. Such sliding bearings have had a serious disadvantage in that there is minimum recovery to an original position or form. If, after encountering a shock or vibration such as produced by an earthquake, original relative positions of structural elements are not quickly reinstated, weakening of the structure and perhaps catastrophic failure may result. The restorative force of such sliding bearings unfortunately is not high.
It has been suggested that displacement control devices could be used to dampen relative movement between structural elements, e.g. a column and a superstructure such as a beam or girder used in a bridge. See e.g. Canadian Patent 1,206,981. Similar such devices have been tested at the National Center for Earthquake Engineering Research at the State University of New York at Buffalo. Results of such a test with respect to a bridge deck are shown in Table 1 below. A shock absorber device, e.g. similar to the one described in Canadian Patent 1,206,981 (a mass energy regulator, MER) was used in combination with a sliding bearing. The combination is referred to herein as a compound bearing. The compound bearing was compared with a traditional lead/rubber bearing and an elastomeric bearing. In Table 1, "X" corresponds to displacement in inches and "g" corresponds to acceleration in g's.
TABLE 1 ______________________________________ Peak compound lead/rubber elastomeric acceleration X g X g X g ______________________________________ 0.2 g 1.7 0.09 2.8 0.11 5.7 0.15 0.4 g 2.4 0.18 5.2 0.24 11.4 0.29 0.6 g 2.7 0.27 7.6 0.36 17.1 0.44 ______________________________________
As can be seen from Table 1, the compound bearing transferred less than about 80% of the g force and between about 35 and 60% of the displacement of a lead/rubber bearing and about 60% of the g force and between about 15 and about 30% of the displacement of an elastomeric bearing. While such compound bearings clearly have merit, significant skill and engineering ability is required to properly install them so that they function properly in conjunction with a support bearing which is still required. Therefore in view of the skill that is required for installation, faulty installation by unskilled workers and even skilled workers is possible and even likely.